1. Field of the Invention
The present invention relates to an electro-photographic photoreceptor having a photoconductive layer on a conductive support via an underlayer, a process for producing the same, and an image-forming apparatus using the same.
2. Description of the Related Art
In a process for electrophotography, one of general information recording methods, utilizing photoconductive phenomena of a photoreceptor, a surface of the photoreceptor is uniformly charged by corona discharge in a dark place, and then the charge of the photo-exposed portion is selectively discharged by image exposure to form an electrostatic latent image at the non-exposed portion. After that, colored charged corpuscles (toner) are adhered to the electrostatic latent image to generate an image as a visual picture. In a sequence of these processes, the followings are requisite basic characteristics of the photoreceptor:
(1) Uniformly chargeable at an appropriate electric potential in a dark place; PA1 (2) Having a potent charge capacity with little discharge in a dark place; and PA1 (3) Superior in photosensitivity to release the electric charge rapidly by photo-irradiation. In addition, high stability and durability are required as follows: readily removable charge from the surface of photoreceptor; small residual electric potential; high mechanical strength; good flexibility; unchangeable electrical characteristics in repeated use, particularly, electrically charged property, photosensitivity and residual electric potential; and durability against heat, light, temperature, humidity and ozone.
In the electrophotographic photoreceptor which has such a structure that a photoreceptive layer is formed as a photoconductive layer over a conductive support, the surface electric charge is microscopically lost or reduced to generate a defect of image because carrier injection is readily caused from the conductive support in the photoreceptive layer. In order to prevent it and improve defective coating and electrically charged property on the surface of conductive support, adhesive property of the photoreceptive layer, and easiness of the application, an underlayer is provided between the conductive support and the photoreceptive layer.
As for adhesive resin materials for forming the underlayer, polyethylene, polypropylene, polystyrene, acrylic resin, vinyl chloride resin, vinyl acetate resin, poly-urethane resin, epoxy resin, polyester resin, melamine resin, silicon resin, poly(vinyl butyral) resin, polyamide resin, copolymer resin containing two or more of their repeating units, casein, gelatin, polyvinyl alcohol, ethylcellulose, and water-soluble polyvinyl acetal resin are included.
Japanese Unexamined Patent Publication (Toku-Kai-Sho; hereinafter referred to as JP-A) 63-178249 discloses a photoreceptor having an intermediate layer between the conductive support and the photoreceptive layer, in which the intermediate layer is composed of a water-soluble polyvinyl acetal resin as a major component, or in which an underlayer consisting of dispersed white pigment such as titanium oxide is provided between the conductive support and the intermediate layer. This intermediate layer is provided for preventing the carrier injection from the conductive support into the photoreceptive layer. The underlayer is provided for preventing interference of light in order to apply the photoreceptor to an image-forming apparatus using a laser light source. In this official gazette, high sensitivity of the photoreceptor is intended to be maintained in repeated use in a condition of lower temperature and lower humidity or of higher temperature and higher humidity. So, electrostatic property of the photoreceptor has been evaluated before and after one-hour exposure is repeated under a variety of environments.
JP-A 6-59489 discloses a photoreceptor in which an underlayer containing as a major component polyvinyl acetal resin soluble in a mixture of water and alcohol is provided between the conductive support and the photoreceptive layer. The underlayer is provided for the purpose of improving adhesion between the conductive support and the photoreceptive layer and maintaining the sensitivity in repeated use. In this official gazette, the electrostatic and image characteristics of the prepared photoreceptor are evaluated before and after the repeated exposure of 10,000 times.
In both of JP-A 63-178249 and JP-A 6-59489, however, increase of the residual electric potential and occurrence of the image defect in repeated use cannot be reduced sufficiently due to single use of the resin.
On the other hand, another underlayer has been disclosed, in which an inorganic pigment is dispersed into the adhesive resin. JP-A 59-93453 discloses an example of the use of surface-treated titanium oxide as an inorganic pigment. According to this official gazette, uneven coating or uneven film thickness caused by rough surface of the conductive support can be avoided without any cutting working or polishing of mirror surface, and dispersibility is increased by treatment of the surface of titanium oxide with a coating agent such as alumina in order to prevent image defect or uneven concentration.
JP-A 63-298251 discloses an example in which a ratio of titanium oxide to an adhesive resin is in a range of 1/1-3/1 by volume. In this official gazette, the content of titanium oxide is optimized intending prolongation of the life span of the photoreceptor, and the electrostatic property of the prepared photoreceptor is evaluated before and after the repeated exposure of 100,000 times.
In JP-A 59-93453 and JP-A 63-298251, increase of the residual electric potential and occurrence of the image defect in repeated use are not reduced sufficiently. Accordingly further improvement is desired.